The present invention relates to transmitting data, and in particular to controlling the load impedance for an amplifier based on the peak-to-average power ratio associated with the data being transmitted.
Mobile terminals, such as wireless telephones, have traditionally transmitted voice or data at very low data rates with an emphasis on optimizing the power added efficiency (PAE). Existing personal communication services (PCS) systems are typically required to communicate using two or more standards. These standards often include communicating using a type of analog frequency modulation (FM) in one mode and a type of amplitude modulation (AM) in another.
Traditionally, the maximum power output requirements for each mode have been different for these dual mode systems. In order to optimize efficiency, load-switching techniques have been used to change the effective load impedance for power amplifiers when operating at the different power levels of the respective modes.
The new Third Generation (3G) code division multiple access (CDMA) PCS system referred to as cdma2000 uses a form of phase shift keying (PSK) modulation for the transmission of both voice and data. Depending on the information content and the rate of transmission, cdma2000 mobile terminals transmit signals of significantly different peak-to-average power ratios at a given output power. For example, voice data typically has a lower peak-to-average power ratio than control information or data signals. In addition, the power amplifier of the mobile terminal is required to meet a certain linearity specification for all signals.
Since the cdma2000 standard is new, PAE and linearity issues for signals of differing peak-to-average power ratios in PCS systems have just surfaced. A power amplifier meeting the linearity requirements for the highest peak-to-average power ratio signal specified has lower PAE for all other signals having a lower peak-to-average power ratio. This lower PAE results in reduced talk time. In addition, power amplifier reliability can be jeopardized by the internal heat generated by the lower PAE for those signals. Conversely, a power amplifier configured to efficiently transmit a signal with lower peak-to-average power ratio cannot meet the requisite linearity to transmit high-speed data.
When optimizing the power amplifier""s linearity for transmitting signals with a higher peak-to-average power, such as data, the power amplifier""s PAE in voice mode (or lower peak-to-average power ratio) is degraded. Given P=V2/RL, where RL represents an overall load impedance, and given that the battery voltage (V) for the mobile terminal cannot increase, RL needs to decrease to meet linearity requirements for the greater peak-to-average power ratio. By lowering RL, the effective saturation power (Psat) of the amplifier is raised, allowing the amplification of signals with a higher peak-to-average power ratio with requisite linearity. Further, the lower RL will not be optimum in PAE for voice transmission, due to the device operating backed off from the optimum point near PSAT.
One technique for measuring linearity of a CDMA power amplifier is to determine the Adjacent Channel Power Ratio (ACPR), which is a measure of the distortion that the power amplifier introduces into an ideal signal. ACPR also determines the amount of interference caused to neighboring channels by power leaking into the adjacent spectrum. These requirements ensure that the transmitted waveform will suffer minimal distortion due to non-linearties in the transmitter channel.
In order to maximize system channel capacity, all wireless standards set limits on adjacent channel leakage. In general, ACPR is determined by:
1) Measuring the average total output at fC within a fBW bandwidth (PTOTAL), where fC is the radio frequency (RF) carrier frequency and fBW is the bandwidth of the modulated carrier as defined by the particular system;
2) Measuring power at xc2x1f1 offset frequency in a fBW1 bandwidth (PACP1), wherein ACPR1 is defined as: ACPR1=PACP1xe2x88x92PTOTAL (dBc), where fBW1 is the bandwidth defined to measure power in at this offset; and
3) Measuring power at xc2x1f2 offset frequency in a fBW2 bandwidth (PACP2), wherein ACPR2 is defined as: ACPR2=PACP2xe2x88x92PTOTAL (dBc) where fBW2 is the bandwidth to defined measure power in at this offset.
More offsets can be specified depending upon the system. Cdma2000 ACPR or linearity requirements are the same regardless of the resultant peak-to-average power ratio, which may greatly vary for both voice data and high-speed data. Since linearity cannot be compromised, the cdma2000 maximum power output requirements for transmitting for modes having a peak-to-average power ratio above a certain level have been lowered in an attempt to meet the linearity requirements for these modes, while maintaining PAE at full power in modes with lower peak-to-average power ratios, such as voice mode. The disadvantage of transmitting control information and high-speed data at a lower output power level is that the range of the mobile terminal is decreased. As the mobile terminal is located further away from a base station, the mobile terminal may be able to communicate in voice mode, but not in certain data modes. Such disparity between voice and high-speed data services is undesirable.
In essence, the rigid linearity requirements are forcing designers to compromise either efficiency for lower peak-to-average power ratio signals or communication range for higher peak-to-average power ratio transmissions, and vice versa. As such, there is a need for a technique to optimize efficiency for transmissions with different peak-to-power ratios, such as those for voice and high-speed data transmissions, while meeting linearity requirements in an economical fashion. There is a further need to provide such operation within a given power level or operating mode.
The present invention relates to controlling load impedance during wireless communications to maintain amplifier linearity for transmissions, such as voice and high-speed data, having significantly different peak-to-average power ratios. At a desired output power, a first load impedance is selected for transmissions having a first peak-to-average power ratio and a second load impedance is selected for transmissions having a second peak-to-average power ratio, in order to ensure that appropriate amplifier linearity is achieved for both voice and high-speed data transmissions. Preferably, amplifier efficiency is optimized for transmissions having the first and second peak-to-average power ratios. Changing the effective load impedance may be effected by providing a first impedance network and switching a second impedance network in association with the first impedance network.
In one embodiment, a switchable impedance network is provided having a first impedance network for transmissions having the first peak-to-average power ratio and a second impedance network, which is switched in parallel with the first impedance network, for transmissions having the second peak-to-average power ratio. The second impedance network is effectively removed from the transmission path during transmissions having the first peak-to-average power ratio and reduces the overall load impedance during transmissions having the second peak-to-average power ratio.
Control of the switchable impedance network is typically provided by a control system using switching circuitry to switch impedance elements into and out of the transmission path. The switching circuitry may take many forms, including switching transistors and PIN diodes.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.